Morphological study of the occipital belly of the occipitofrontalis muscle and its innervation

Purpose: Occipital belly (OB) of occipitofrontalis muscle (epicranius) is a muscle which covers the occipital part of the skull. The posterior auricular nerve (PAN) is the first extracranial branch of the facial nerve, which supplies the OB. The PAN is one of the structures used to identify the facial nerve during surgeries such as parotidectomy and mastoidectomy. In the present report, we provide detailed anatomical knowledge of the OB and its innervation. Methods: Twenty-six hemifaces from 14 Korean cadavers were dissected. The mastoid tip, external occipital protuberance (EOP), a horizontal line that is parallel to the Frankfurt horizontal plane (x-axis), and a vertical line through the EOP (y-axis) were used as reference points and lines. Results: The OB demonstrated a variety of features and was mostly asymmetrical. The muscle bellies were observed to angle toward the temporoparietalis muscle laterally, with the aponeurosis angled at approximately 55°–65°. The mean width and height were 60.9 ± 8.7 and 31.7 ± 7.5 mm, respectively. Muscle bellies were located at a mean distance of 7.1 ± 2.5 mm superior to the x-axis and 29.6 ± 6.4 mm lateral to the y-axis. The mean vertical distance from the origin of the PAN at the anterior border of the mastoid process (MP) to the mastoid tip was 6.1 ± 2.1 mm. The mean nerve angle between the PAN and the x-axis was 55.7° ± 6.8°. The entry point of the PAN that innervates the OB was positioned at a mean distance of 9.0 ± 3.5 mm superior to the x-axis and 79.0 ± 8.1 mm lateral to the y-axis. Conclusions: Understanding the morphometrical characteristics of the OB and its innervation may potentially improve surgical outcomes to assist in locating the posterior auricular branch of the facial nerve.